Zeolite catalysts are used in various processes such as DTO reaction/MTO reaction for synthesizing lower hydrocarbons from dimethyl ether (hereafter referred to as DME) and/or methanol, MTG reaction for synthesizing gasoline from methanol, fluid catalytic cracking (FCC) or the like.
In these processes, deactivation of zeolite catalysts may occur. For example, the following is the main reason for deactivating zeolite catalysts. Where the zeolite catalyst is exposed to the reaction atmosphere containing steam, elimination of aluminum from the zeolite framework (dealumination) may occur. In addition, carbonaceous deposits are formed on the zeolite catalysts during the reaction.
Reduction of catalytic activity caused by the formation of carbonaceous deposits on the catalyst may be solved by providing a flow containing oxygen to the catalyst and burning the carbonaceous deposits on the catalyst. On the other hand, as a countermeasure for reduction of catalytic activity caused by the elimination of aluminum from the zeolite framework, a method is proposed for inserting aluminum into the framework by treating the dealuminated zeolite in particular conditions.
As methods for regenerating a dealuminated zeolite, a method for regenerating the zeolite using aluminum chloride and acid (for example, Patent Reference 1: Japanese Unexamined Patent Application, First Publication No. S59-136138), and a method for regenerating the zeolite using steam and ammonia are disclosed (for example, Patent Reference 2: Japanese Unexamined Patent Application, First Publication No. S60-257838).
In addition, a method for inserting aluminum into a high-silica zeolite by compounding a high-silica zeolite with alumina (aluminum oxide) and treating the compounded material with steam is known (for example, Patent Reference 3: Japanese Examined Patent Application, Second Publication No.143-63430, Patent Reference 4: U.S. Pat. No. 4,559,314, Patent Reference 5: U.S. Pat. No. 4,784,747, Patent Reference 6: Japanese Patent, No. 2908959, Non Patent Reference 1: J. Catal., 93, 471 (1985), Non Patent Reference 2: J. Chem. Soc. Faraday Trans. 1, 81, 2215 (1985)).
However, the method for regenerating the dealuminated zeolite included a disadvantage in the applicability to industrial processes because of requirements for specific reagents or gas for the regeneration.
In addition, although the above-described method to compound the high-silica zeolite with alumina and to treat the compounded material with steam was examined on the zeolite containing a small amount of aluminum, for example a zeolite having a molar ratio of Si/Al>1200. The method was not examined on the zeolite having a molar ratio of Si/Al from several tens to ca. 300, which was frequently used in industrial processes.
On the other hand, it is known that the formation rate of carbonaceous deposits on the zeolite catalyst can be decreased by a proper steam treatment. In a disclosed method, catalytic lifetime for synthesizing hydrocarbons from methanol is increased by exposing MFI-structure zeolite catalyst to steam thereby controlling acid sites (active sites) of the zeolite (e.g., Patent Reference 7: U.S. Pat. No. 4,429,176, Patent Reference 8: U.S. Pat. No. 4,663,942, Patent Reference 9: U.S. Pat. No. 4,579,993). In addition, it is found that lifetime of an alumina-containing zeolite catalyst is prolonged by exposing the alumina-containing zeolite catalyst to steam thereby decreasing the coking rate of the catalyst (e.g., Patent Reference 10: U.S. Pat. No. 4,456,780). However, it has been unknown how the addition of alumina change the steam resistance of zeolite catalysts. In addition, there is no report treating a catalyst containing zeolite, alumina, and alkaline-earth metal compound with steam.
In DTO reaction/MTO reaction, it is disclosed that by using a proton-type MFI-structure zeolite impregnated with alkaline-earth metal compound, selectivity to lower olefins is increased, formations of paraffins and aromatic hydrocarbons are depressed, and formation of carbonaceous deposits is depressed, thereby prolonging lifetime of the catalyst (e.g., Patent Reference 11: Japanese Unexamined Patent Application, First Publication, No. S60-126233). However, in Patent Reference 11, it was not examined if lifetime of the zeolite catalyst modified with alkaline-earth metal compound was changed (or is not changed) by repeating regeneration of the catalyst after performing DTO reaction/MTO reaction In addition, steam resistance of the catalyst is not described in Patent Reference 11.
As a representative example of catalyst used for DTO reaction/MTO reaction, MFI-structure zeolite catalysts and SAPO-34 catalysts may be used.
In the DTO reaction/MTO reaction, activity of the catalyst is decreased by formation of carbonaceous deposits on the catalyst. Therefore, it is necessary to periodically introduce an oxygen-containing flow to the catalyst so as to burn the carbonaceous deposits on the catalyst, thereby regenerating the catalytic activity.
The combustion reaction to burn the carbonaceous deposits on the catalyst is an exothermal reaction. So as to prevent changing the catalyst such as collapse of crystal structure, and for a stable operation of apparatus used in the process, it is preferable to inhibit large increase of temperature. Therefore, in the above-described combustion reaction, so as to depress oxygen concentration to a lower level, the air introduced to the catalyst must be diluted by inert gas such as steam and nitrogen.
However, steam promote the elimination of aluminum from the zeolite catalyst to lead decreasing catalyst lifetime. Therefore, there is a method for using nitrogen as the dilution gas to keep the steam concentration at low level (e.g., U.S. Published Patent Application No, 2005/0085375)
It is difficult to apply the above-described method of regenerating dealuminated zeolite in industrial processes. In addition, the method includes a problem that an extra step is needed for inserting aluminum into the dealuminated zeolite. Therefore, in order to improve the lifetime of zeolite catalyst, it is necessary to produce a zeolite catalyst in which the framework aluminum is not likely to be eliminated.
On the other hand, when nitrogen gas is used to dilute the oxygen concentration in the regeneration atmosphere, a cryogenic air separator for producing the nitrogen gas is required, thereby increasing construction cost of a plant.
Based on the above described consideration, an object of the present invention is to provide an alkaline-earth metal compound-containing zeolite catalyst, in which the elimination of tetrahedral aluminum from the zeolite framework is not likely to occur, and a simple inexpensive method for preparing the above-described zeolite catalyst. Another object of the present invention is to provide a method for regenerating alkaline-earth metal compound-containing zeolite catalyst, by which method, catalytic activity of the alkaline-earth metal compound-containing zeolite catalyst is regenerated through a simple process, and lifetime of the catalyst is improved.